A curve shown in FIG. 9 indicates the rate of failures occurring with time in an LSI. This curve, similar in shape to a bathtub, is called a bathtub curve. Failures in an LSI are divided into early failures, random failures, and wear-out failures.
Conventional LSIs are inspected prior to shipment so as to ensure the quality thereof. Such pre-shipment inspection is performed after the LSIs are put under load which is heavier than specified by the specifications in terms of temperature and voltage conditions, such that products in which early failures will occur over time are prevented from appearing on the market.
With such pre-shipment inspection, however, it has been difficult to deal with failures that could occur in an LSI shipped as a finished product to be incorporated into a system and continuously used in the system, in particular, failures corresponding to wear-out failures in a bathtub curve.
In addition, as the degree of integration in LSIs has been increased, each LSI has become multifunctional. This may result in a situation in which one functional circuit in an LSI has no degradation failure, but another functional circuit suffers a degradation failure.
In order to address these problems, a conventional semiconductor device incorporates two or more functional circuits, performs self-diagnosis, and uses one of the functional circuits that has no fault, thereby extending the life of the LSI (see Patent Document 1, for example).
There is another semiconductor device, in which self-diagnosis is performed on a functional circuit, and supply of a clock is stopped to halt the operation of the functional circuit (see Patent Document 2, for example).    Patent Document 1: Japanese Laid-Open Publication No. 2004-340877    Patent Document 2: Japanese Laid-Open Publication No. 05-341013